In heavy earth moving equipment, such as draglines or power shovels for mining, an upper structure is rotatably supported on a lower structure by a roller circle assembly. A boom typically extends from the upper structure, and a sheave is placed on the upper end of the boom. A hoist rope extends generally downwardly from the sheave to a bucket or dipper, depending on the application. In the dragline application, the bucket is also connected to the upper structure by a drag rope. Both the hoist rope and the drag rope can control the movement of the bucket. The lower structure of the dragline typically is mounted on treads or a walking tub mechanism for movement over the ground.
The roller circle assembly defines an axis of rotation for the upper structure relative to the lower structure. A lifting pin can extend along the axis and connect the upper structure to the lower structure. A pinion gear drive may also be mounted on the upper structure for engagement with a ring gear mounted on the lower structure that operates to rotate the upper structure relative to the lower structure. The roller circle may include an upper circular rail coupled to the upper structure, a lower circular rail coupled to the lower structure, and rollers positioned between the circular rails. The rails can be in segments. The rollers are typically tapered, having their larger diameter located radially outward.
Roller circle assemblies in heavy earth moving equipment can measure from three meters to more than 20 meters in diameter. Very heavy loads, amounting to several hundred metric tons, may be transmitted through the bearing assemblies to the lower structure, and the overturning moments created by loads on the boom can create highly concentrated stresses on small segments of the bearing assembly. A misalignment between the upper and the lower structures can further contribute to highly concentrated stresses. This misalignment can be due to a gap around the lifting pin that is created for built-in tolerance or through wear. Moreover, the loading along a given arc length of the roller circle can be non-uniform during operations such as digging, dumping, transferring, and so forth. Under dynamic non-uniform loading, the rollers of the roller circle assemblies may twist or distort relative to the rails, further concentrating the load on a relatively small contact area and bringing about costly failures.